This proposal examines the effects of chronic stress on the balance between multiple memory systems. Chronic stress impairs learning and memory on many tasks, though there are some reports of no effect or enhanced learning after chronic stress. Current findings indicate that different memory systems interact in a manner that includes instances of competition, often demonstrated by showing that lesions or pharmacological inactivation of one system can enhance learning of a task typically associated with a different neural system. By using in vivo microdialysis coupled to sensitive assay procedures, it is possible to examine release of acetylcholine during the course of training rats in mazes, with sample collection timed to match only a few training trials. Findings from studies such as these have shown that release of acetylcholine in several neural systems is a useful marker of the extent to which a system is activated by training. For example, when rats are trained on a T-maze that can be learned using either place (hippocampus-dependent) or response (striatum-dependent) solutions, acetylcholine release before and during training predicts which rats use each solution. This proposal will use such procedures to determine whether chronic stress produces a shift in which neural system is preferentially activated during learning, thereby impairing learning of some tasks and enhancing learning of other tasks. In addition, this proposal will examine simultaneously acetylcholine release in the hippocampus and striatum, at baseline and in response to training, to determine whether chronic stress results in different profiles of acetylcholine release associated with changes in cognitive processing. Thus, the findings of these experiments may offer a basis for apparently discrepant behavioral findings after chronic stress. More generally, the findings address the key issue of whether chronic stress results in a change in the balance between neural systems, thereby altering the strategy selected by a rat faced with a new problem to solve.